Forced expiration is a simple yet useful test for pulmonary function. In this test, the patient makes a maximum inspiration, filling the lungs to their maximum volume known as Total Lung Capacity (TLC), and immediately expels the inhaled air as rapidly as possible to empty the lungs to the minimum volume attainable by this maneuver. By measuring the volume rate of expiration and the total amount of air expired over time, a number of parameters can be used as indications of pulmonary function and health. Examples of these parameters include the forced expiratory flow in the first second of the forced expiration (FEV1), the expiratory flow rate averaged over the period from a volume of 25% to 75% of the total expired air volume (FEV25-75), the total volume of expired air (FVC) plotted against the flow rate. The forced expiration test is useful for quantitating levels of dysfunction occurring in relation to both obstructive and restrictive pulmonary processes.
In infants, lung function tests that evaluate airway function is particularly difficult as they cannot cooperate for the test. Unlike an older child or an adult, an infant cannot be expected to voluntarily perform the forced expiration on his own. Hence, to acquire data on an infant's airways, clinicians have developed techniques such as the “squeeze” or “hug” technique. In this method, an inflatable vest or jacket is wrapped around the infant's chest and abdomen. The vest is attached to a pressurized reservoir with a valve controlling the flow of air from the reservoir into the vest. With the reservoir, the vest can be rapidly filled with air (to varying pressures) to squeeze the infant's body within the vest and expel the air from the infant's lungs so as to replicate the forced flow procedure. Because these squeezes are made from end-tidal inspiration, the infant is not inflated to TLC before the squeeze resulting in partial respiratory curves.
One way to acquire more information across a greater exhale volume of the infant is to inflate the infant's lung to a higher lung volume before performing the squeeze. For example, infant lungs can be inflated to +15 to +50 cm H2O before squeezing or deflation that forces the air out. See e.g., Motoyama, E K, Pulmonary Mechanics During Early Post Natal Years, Pediatric Respiration, Vol. 11, 1977, pp. 220–223; Turner, D J et al., Assessment of Forced Expiratory Volume-Time Parameters In Detecting Histamine Induced Bronchoconstriction in Wheezy Infants, Pediatric Pulmonology, Vol. 15, No. 4, pp. 220–4, April 1993; Hammer J. and Newth, C J., Effect of Lung Volume on Forced Expiratory Flows during Rapid Thoracoabdomical Compression in Infants, Journal of Applied Physiology, Vol. 78, No. 5, 1995, pp. 1993–1997; Newth, C J et al., The Effects of Varying Inflation and Deflation Pressures on the Maximal Expiratory Deflation Flow-Volume Relationship in Anesthetized Rhesus Monkeys, American Review of Respiratory Disease, Vol. 144, No. 4, pp. 807–13, October 1991. Lung volume may also be increased by providing for multiple inspiration without exhalation. Because under these methods, more air is expelled than during a normal breath, the time for exhalation is prolonged beyond what the infant's brain determines the time should be. In response to the innate timing cycle, however, the infants frequently begin inspiration before the forced maneuver is completed, and invalidates the data.
One way to address this problem was for the physician to wait until a respiratory pause has been achieved in the infant before “squeezing” or forcing the infant to exhale. In this method, the physician first inflates the infant's lung synchronously with the infant's natural tidal inspiration to a lung volume greater than the lung volume reached with natural breathing for several breaths. Inflating the infant's lungs in this way will cause a few seconds of pause in the infant's breathing. During this pause period, the infant's lung is again rapidly inflated and immediately afterwards, the chest and abdomen is “squeezed” to produce a maximum forced expiration. In the above method, however, the infant's carbon dioxide levels in the blood, which controls their innate respiratory timing, will be returning to normal while the clinician is waiting to determine if there has been a respiratory pause. Carbon dioxide levels returning to normal may affect the test results. Accordingly, there is a continuing need for new methods of performing forced expiratory maneuvers in infants.